EXHAUST PURIFICATION DEVICE FOR SPARK IGNITION INTERNAL COMBUSTION ENGINE

摘要

A NOx storage catalyst (22) and a NOx selective reduction catalyst (23) are arranged in the exhaust passage of an internal combustion engine. In a low-load engine operation region, combustion is performed in the combustion chamber (5) on the basis of a base air-fuel ratio that is lean, and when NOx must be released from the NOx storage catalyst (22), the air-fuel ratio in the combustion chamber (5) is made rich. In a medium-load engine operation region, the base air-fuel ratio is reduced and the degree of rich of the air-fuel ratio is reduced, thereby reducing the period in which the air-fuel ratio is rich, such that the amount of ammonia generated per unit time when the air-fuel ratio is made rich is lower than for the low-load engine operation region.